Unlike P-51B/C production, where the identical products had different designators based on place of manufacture, the Inglewood and Dallas plants both produced P-51D's. The only thing different between them was the suffix - for instance, an Inglewood-produced P-51D-10-NA was the equivalent to a Dallas-produced P-51D-10-NT. Both facilities had great demand for the Hamilton Standard propellers that were installed on the P-51D (and the B/C's before it), and Hamilton Standard wasn't able to keep up with demand (NAA, through both plants, could produce more P-51D's than propeller units could be provided). As a result, Aeroproducts was contracted to supply an alternative option for the Dallas plant. The Aeroproducts propellers required a new nose cone assembly to be designed and manufactured. This was the only production change from the P-51D, but it resulted in the new designator P-51K. From what I understand, the production of P-51K's was directly interwoven within the manufacture of P-51D's at the Dallas plant - for instance, within the P-51D-5-NT production block, there was a production block of P-51K-1-NT's, and within the P-51D-10-NT production block, there was a production block of P-51K-5-NT's. The difference in suffix was the result of the fact that full P-51D production started with the P-51D-5's (P-51D-1's were the few prototypes), while full P-51K production started with P-51K-1, thus the offset nature of the equivalent production block numbers.
The only other recognizable difference, although not intended, was in the canopy shape - with a different company supplying the canopies for the Dallas production, using a different production method, the canopies on the Dallas-produced P-51D's and P-51K's (both early and later design) took on a slightly different form to those produced for the Inglewood production, looking more 'slicked-back'.
There wasn't anything to prevent the nose cones/prop assemblies from being interchanged between P-51D's and P-51K's, and, based on available supplies, some P-51K's wound up being fitted with Hamilton Standard propellers while in operation.
Taken from the P-51D/K E&M, here is the only production difference between the two, clearly drawn:
Hamilton Standard propeller and nose cone:
Aeroproducts propeller and nose cone: